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Processes and Principles of Erosion and Sedimentation

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Processes and Principles of Erosion and Sedimentation 2 Processes and Principles of Erosion and Sedimentation Processes and Principles of Erosion and Sedimentation 2 Processes and Principles of Erosion and Sedimentation When land is disturbed at a construction site, the erosion rate accelerates dramatically. Since ground cover on an undisturbed site protects the surface, removal of that cover increases the site’s susceptibility to erosion. Disturbed land may have an erosion rate 1,000 times greater than the pre-construction rate. Even though construction requires that land be disturbed and left bare for periods of time, proper planning and use of control measures can reduce the impact of man-induced accelerated erosion. The major problem associated with erosion on a construction site is the movement of soil off the site and its impact on water quality. Millions of tons of sediment are generated annually by the construction industry in the United States. The rate of erosion on a construction site varies with site conditions and soil types but is typically 100 to 200 tons per acre and may be as high as 500 tons per acre. In N.C., 15% to 32% of eroded soil is transported to valuable water resources (SCS, 1977). Identifying erosion problems at the planning stage and noting highly erodible areas, helps in selecting effective erosion control practices and estimating storage volumes for sediment traps and basins. This manual focuses primarily on the prevention of sedimentation problems associated with water-generated soil erosion. THE EROSION AND SEDIMENTATION PROCESS Types of Erosion Erosion is a natural process by which soil and rock material is loosened and removed. Erosion by the action of water, wind, and ice has produced some of the most spectacular landscapes we know. Natural erosion occurs primarily on a geologic time scale, but when man’s activities alter the landscape, the erosion process can be greatly accelerated. Construction-site erosion causes serious and costly problems, both on-site and off-site. 2.1 2 7KHVRLOHURVLRQSURFHVVEHJLQVE\ZDWHUIDOOLQJDVUDLQGURSVDQGÀRZLQJRQ the soil surface. Figure 2.1 illustrates the four types of soil erosion on exposed terrain: splash, sheet, rill, and gully, and stream and channel. Splash erosion results when the force of raindrops falling on bare or sparsely vegetated soil detaches soil particles. Sheet erosion occurs when these soil particles are HDVLO\WUDQVSRUWHGLQDWKLQOD\HURUVKHHWE\ ZDWHUÀRZLQJ,I WKLVVKHHW runoff is allowed to concentrate and gain velocity, it cuts rills and gullies as it GHWDFKHVPRUHVRLOSDUWLFOHV$VWKHHURVLYHIRUFHRIÀRZLQJZDWHULQFUHDVHV with slope length and gradient, gullies become deep channels and gorges. The JUHDWHUWKHGLVWDQFHDQGVORSHWKHPRUHGLI¿FXOWLWLVWRFRQWUROWKHLQFUHDVLQJ volume and velocity of runoff and the greater the resultant damage. Sedimentation Sedimentation is the deposition of soil particles that have been transported by water and wind. The quantity and size of the material transported increases with the velocity of the runoff. Sedimentation occurs when the water in which WKHVRLOSDUWLFOHVDUHFDUULHGLVVXI¿FLHQWO\VORZHGIRUDORQJHQRXJKSHULRG of time to allow particles to settle out. Heavier particles, such as gravel and VDQGVHWWOHRXWVRRQHUWKDQGR¿QHUSDUWLFOHVVXFKDVFOD\7KHOHQJWKRI time a particle stays in suspension increases as the particle size decreases. The colloidal clays stay in suspension for very long periods and contribute VLJQL¿FDQWO\WRZDWHUWXUELGLW\ 2.2 Processes and Principles of Erosion and Sedimentation Factors that The potential for an area to erode is determined by four principal factors: soils, surface cover, topography, and climate. These factors are interrelated in their ,QÀXHQFH(URVLRQ effect on erosion potential. The variability in North Carolina’s terrain, soils, and vegetation makes erosion control unique to each development. Understanding the factors that effect the erosion process enables us to make useful predictions about the extent and consequences of on-site erosion. An empirical model developed for agricultural applications, the Universal Soil Loss Equation (USLE), predicts soil loss resulting from sheet and rill erosion. It considers both the effects of erosion control practices and the factors that LQÀXHQFHHURVLRQVRLWLVXVHIXOIRUHYDOXDWLQJHURVLRQSUREOHPVDQGSRWHQWLDO VROXWLRQV7KHIDFWRUVWKDWLQÀXHQFHHURVLRQDUHVRLOFKDUDFWHULVWLFVVXUIDFH cover, topography, and climate. Soils A soil is a product of its environment. The vulnerability of a soil to erosion, known as its erodibility, is a result of a number of soil characteristics, which FDQEHGLYLGHGLQWRWZRJURXSVWKRVHLQÀXHQFLQJLQ¿OWUDWLRQWKHPRYHPHQW of water into the ground; and those affecting the resistance to detachment and transport by rainfall and runoff. The soil erodibility factor (K) is a measure of a soil’s susceptibility to erosion by water. Key factors that affect erodibility are soil texture, content of organic matter, soil structure, and soil permeability. Soil texture is described by the proportions of sand, silt, and clay in the soil. +LJK VDQG FRQWHQW JLYHV D FRDUVH WH[WXUH ZKLFK DOORZV ZDWHU WR LQ¿OWUDWH UHDGLO\ UHGXFLQJ UXQRII $ UHODWLYHO\ KLJK LQ¿OWUDWLRQ UDWH FRXSOHG ZLWK resistance to transport by runoff results in a low erosion potential. Soils FRQWDLQLQJKLJKSURSRUWLRQVRIVLOWDQGYHU\¿QHVDQGDUHPRVWHURGLEOH&OD\ acts to bind particles and tends to limit erodibility; however, when clay erodes, the particles settle out very slowly. Because organic matter, such as plant material, humus, or manure, improves soil structure, increases water-holding capacity, and may increase the LQ¿OWUDWLRQUDWHLWUHGXFHVHURGLELOLW\DQGWKHDPRXQWRIUXQRII Soil structure is determined by the shape and arrangement of soil particles (Figure 2.2). A stable, sharp, granular structure absorbs water readily, resists HURVLRQE\VXUIDFHÀRZDQGSURPRWHVSODQWJURZWK&OD\VRLOVRUFRPSDFWHG VRLOVKDYHVORZLQ¿OWUDWLRQFDSDFLWLHVWKDWLQFUHDVHUXQRIIUDWHDQGFUHDWHVHYHUH erosion problems. Soil permeability refers to a soil’s ability to transmit air and water. Soils that are least subject to erosion from rainfall and shallow surface runoff are those with high permeability rates, such as well-graded gravels and gravel- sand mixtures. Loose, granular soils reduce runoff by absorbing water and by providing a favorable environment for plant growth. Surface Cover Vegetation is the most effective means of stabilizing soils and controlling HURVLRQ,WVKLHOGVWKHVRLOVXUIDFHIURPWKHLPSDFWRIIDOOLQJUDLQUHGXFHVÀRZ YHORFLW\DQGGLVSHUVHVÀRZ9HJHWDWLRQSURYLGHVDURXJKVXUIDFHWKDWVORZV WKHUXQRIIYHORFLW\DQGSURPRWHVLQ¿OWUDWLRQDQGGHSRVLWLRQRIVHGLPHQW 2.3 2 Plants remove water from the soil and thus increase the soil’s capacity to absorb water. Plant leaves and stems protect the soil surface from the impact of raindrops, and the roots help maintain the soil structure. 7KH W\SH DQG FRQGLWLRQ RI JURXQG FRYHU LQÀXHQFH WKH UDWH DQG YROXPH RI runoff. Although impervious surfaces protect the area covered, they prevent LQ¿OWUDWLRQDQGWKHUHE\GHFUHDVHWKHWLPHRIFRQFHQWUDWLRQIRUUXQRII7KH UHVXOWLVKLJKSHDNÀRZDQGLQFUHDVHGSRWHQWLDOIRUVWUHDPDQGFKDQQHOHURVLRQ (Figure 2.3). Nonvegetative covers such as mulches, paving, and stone aggregates also protect soils from erosion. Topography 7RSRJUDSKLFIHDWXUHVGLVWLQFWO\LQÀXHQFHHURVLRQSRWHQWLDO:DWHUVKHGVL]H and shape, for example, affect runoff rates and volumes. Long, steep slopes LQFUHDVHUXQRIIÀRZYHORFLW\6ZDOHVDQGFKDQQHOVFRQFHQWUDWHVXUIDFHÀRZ which results in higher velocities. Exposed south-facing soils are hotter and GULHUZKLFKPDNHVYHJHWDWLRQPRUHGLI¿FXOWWRHVWDEOLVK Climate North Carolina has considerable diversity of climate. A hurricane season along the coastal region and snow and ice in the mountains are examples of the extremes in weather. High-intensity storms that are common in North Carolina produce far more erosion than low-intensity, long-duration storms with the same runoff volume. The frequency, intensity, and duration of rainfall and the size of the area on which the rain falls are fundamental factors in determining the amount RI UXQRII SURGXFHG 6HDVRQDO WHPSHUDWXUH FKDQJHV DOVR GH¿QH SHULRGV RI high erosion risk. For example, precipitation as snow creates no erosion, but repeated freezing and thawing breaks up soil aggregates, which can be transported readily in runoff from snowmelt. 2.4 Processes and Principles of Erosion and Sedimentation Impacts of Erosion Damage from sedimentation is expensive both economically and and Sedimentation HQYLURQPHQWDOO\ 6HGLPHQW GHSRVLWLRQ GHVWUR\V ¿VK VSDZQLQJ EHGV reduces the useful storage volume in reservoirs, clogs streams, may carry WR[LFFKHPLFDOVDQGUHTXLUHVFRVWO\¿OWUDWLRQIRUPXQLFLSDOZDWHUVXSSOLHV Suspended sediment can reduce in-stream photosynthesis and alter a stream’s ecology. Many environmental impacts from sediment are additive, and the ultimate results and costs may not be evident for years. The consequences of off-site sedimentation can be severe and should not be considered as just a problem to those immediately affected. On-site erosion and sedimentation can cause costly site damage and construction delays. Lack of maintenance often results in failure of control practices and expensive cleanup and repairs. PRINCIPLES OF EROSION AND SEDIMENTATION CONTROL (IIHFWLYH HURVLRQ DQG VHGLPHQWDWLRQ FRQWURO UHTXLUHV ¿UVW WKDW WKH VRLO surface be protected from the erosive forces of wind, rain, and runoff, and second that eroded soil be capture on-site. The following principles are not complex but are effective. They should be integrated into a system of control measures and management techniques to control erosion and prevent
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